References

6TargetsofSI 81
al. 2002). Thus, SI induces massive and sustained F-actin depolymerization
in incompatible pollen.
6.2.4
Increases in Cytosolic Calcium Lead to Changes in F-Actin
As we had established that Ca 2+ acts as a second messenger during the SI response
(Franklin-Tong et al. 1993, 1997), we wondered whether F-actin was
atargetforCa 2+ -dependent signalling. We therefore investigated whether
increasing [Ca 2+ ]i in pollen tubes, using the Ca 2+ ionophore A23187 and
mastoparan (Franklin-Tong et al. 1993, 1996), stimulated changes in Factin.
These treatments triggered reorganization of the F-actin cytoskeleton
that appeared broadly similar to those stimulated by SI and led to quantitative
reductions in F-actin levels (Snowman et al. 2002). This suggests that
during SI increases in [Ca 2+ ]i perturb the actin cytoskeleton.
6.2.5
Profilin and Gelsolin: Mediators of Actin Alterations?
ThemarkedchangestotheactincytoskeletonintheSIresponseraisedthe
question of what molecular machinery transduces the changes in [Ca 2+ ]i
into destruction of the F-actin. ABPs are stimulus-response modulators of
actin cytoskeleton reorganization (reviewed by Holdaway-Clarke and Hepler
2003; Staiger and Hussey 2004). F-actin depolymerization could result
from a loss of side-binding proteins, capping of filament ends, stimulation
of filament severing, increased activity of a sequestering protein, or a combinationoftheseactivities.However,fewABPsinpollentubeshavebeen
well characterized (reviewed by Hepler et al. 2001; McCurdy et al. 2001;
Staiger and Hussey 2004). Because actin depolymerization can be achieved
using calcium ionophores, we attempted to model this in vitro with Ca 2+ -
sensitive ABPs. Although plant ADF/cofilin can cause actin depolymerization
and is regulated by Ca 2+ indirectly, increases in [Ca 2+ ]i are expected to
inhibit the F-actin depolymerizing activity of ADF (Smertenko et al. 1998,
Allwood et al. 2001), so its involvement in SI has not been examined.
Pollen profilin exhibits increased actin-sequestering activity at elevated,
but physiologically relevant, Ca 2+ concentrations (Kovar et al. 2000). A large
pool of profilin was predicted to buffer G-actin and maintain the low actin
polymer level in pollen (reviewed by Staiger and Hussey 2004). However,
experiments with native Papaver pollen profilin failed to account for the
high level of depolymerization observed during SI (Snowman et al. 2002).
We proposed that other ABPs may function in concert with profilin to
achieve sustained actin depolymerization during SI.